The radiative component of the Universe has a characteristic impact on both large scale structure ( LSS ) and the cosmic microwave background radiation ( CMB ) .
We use the recent WMAP data , together with previous CBI data and 2dF matter power spectrum , to constrain the effective number of neutrino species N _ { eff } in a general Cosmology .
We find that N _ { eff } = 4.31 with a 95 per cent C.L . 1.6 \leq N _ { eff } \leq 7.1 .
If we include the H _ { 0 } prior from the HST project we find the best fit N _ { eff } = 4.08 and 1.90 \leq N _ { eff } \leq 6.62 for 95 per cent C.L .
The curvature we derive is still consistent with flat , but assuming a flat Universe from the beginning implies a bias toward lower N _ { eff } , as well as artificially smaller error bars .
Adding the Supernovae constraint doesn ’ t improve the result .
We analyze and discuss the degeneracies with other parameters , and point out that probes of the matter power spectrum on smaller scales , accurate independent \sigma _ { 8 } measurements , together with better independent measurement of H _ { 0 } would help in breaking the degeneracies .